Accelerating rates of species extinction and invasion have sparked recent interest in how changes in plant community composition can be propagated through food webs. Research in this area has, however, been largely restricted to considerations of how detrital species mixing affects litter decay processes. The consequences of changing detrital resources for whole assemblages of sediment-dwelling invertebrates remain largely unknown. We manipulated the availability of three detrital sources, Avicennia marina leaves, Posidonia australis blades and Sargassum sp. thalli, on an Australian mudflat to test hypotheses about how changes in the type and number of macrophytes contributing to detrital resources might impact benthic invertebrate assemblages of estuarine soft-sediments. By controlling for changes in total detrital biomass and ensuring that each detrital source was present in two- and three-species mixes as well as monocultures, our experimental design was able to distinguish among effects of mixing, identity and biomass. Three months after detrital manipulation, macroinvertebrate abundance and species richness differed among treatments according to the biomass of detritus added and non-additive effects of detrital species mixing. Whereas the mixing of two detrital species generally had an antagonistic effect on macroinvertebrate abundance and richness, faunal assemblages did not appreciably differ between three-species mixes and monocultures. Generally negative effects of two-species mixes on macroinvertebrates were opposed by positive effects on microphytobenthos, an important food-source for many of the animals. Non-additive effects on sediment communities were particularly apparent when Sargassum sp., the most labile of the three detrital sources considered, was included in two-species mixes. This demonstration of non-additive and identity-dependent effects of detrital species mixing on soft-sediment communities suggests that predicted compositional changes to aquatic macrophyte communities, resulting from coastal development and climate change, will flow on to effect other components of the estuarine food-web.